I'm hot-rodding a Toyota 4AGZE, which is a 1600cc 4-valve supercharged motor.  I would like to test the exhaust sealing ability of the valves by heating a closed valve with a propane torch (and then pressurizing the valve from the back side).  Can someone give me a ball-park figure for the normal operating temperature range for an exhaust valve in such a motor?  Probably the operating range of an exhaust valve(API Cast Steel Valves) for a generic IC gasoline engine would be close enough for what I am doing. 

It seems to me what you are trying to do is a bit counter intuitive.  To start, I doubt you will be able to get an exhaust valve, assembled in a cylinder head, anywhere near hot enough with a "propane torch"...An oxy-acetylene torch, perhaps. Second, you are "pressurizing" the ex valve from the wrong side.  My question is WHY?  There are several other, more conventional testing methods to check the valve seal...comp ck, leak down and, even just sticking a vacuum cleaner hose in the ex port and using a stethoscope to listen for leaks.  As primitive as that sounds, I find it more reliable than "pressurizing the valve from the back side".

The primary path for heat transfer in an exhaust valve is through its contact with the valve seat.  And since your exhaust valve is an axisymmetric part with a relatively uniform input of heat flux, it should not experience much distortion with temperature.  The heat input into the valve varies during an engine cycle, due to things like exhaust gas pressure and temperature, intake/exhaust flow overlap, A/F ratios, and engine load.  

The leakage rate of your seated exhaust valve will be more affected by the heat transfer rate between the seat and head than anything else.  With a small valve head diameter, overheating should not be a problem.  Your exhaust valves should not see temps higher than about 1300degF, even with high levels of supercharge.  But if you want to check exhaust valve leakage at operating temps, you will need to make sure that the valve, seat, head, etc. are all at the correct temperature, not just the exhaust valve.

Additionally, pressure leakage past the piston rings is likely much more detrimental than any leakage past the exhaust valve seats.  So maybe you're efforts might be misplaced.

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